1. Field of the Invention
The present invention relates to an information processing system in which computers are connected to respective disk devices.
2. Description of Related Art
In a conventional computer system, multiple computers were interconnected via a LAN or other network and disk devices were directly connected to the computers. The data stored in the disk devices were managed by the computers that were directly connected to them. In other words, the data were distributed for management purposes.
Recently, however, there is an increase in the number of configurations in which multiple computers share a disk device, including a configuration for sharing input/output channels for virtual computing and computer logical sharing, a storage area network (hereinafter abbreviated to “SAN”), and a network attached storage (hereinafter abbreviated to “NAS”). These configurations offer the advantage of reducing the cost of disk devices as well as the cost of management, because they make it possible to integrate the data distributed to multiple disk devices for centralized management purposes.
The operating system function called “Union File Mount”, which is stated on pages 235 through 237 of Addison Wesley's “The Design and Implementation of the 4.4BSD Operating System (1996)”, provides a means of partial sharing by saving the updates for a file system established on a disk device onto another disk device.
All the above-mentioned conventional technologies provide a means of sharing user disks and a part of system disk, but do not permit the entire system disk to be shared. The reason is that individual computers have their own unique items such as a configuration file.
When, for instance, a NAS or other network file system is used, flexible operations cannot be performed because there is no alternative but to define common and private parts in the unit of a directory and only partial sharing is achievable.
The “Union File Mount” function, on the other hand, permits partial sharing. However, the function does not become available until the operating system is completely started up because it relates to the file system, which is an upper layer of the operating system. Therefore, it cannot be applied to the boot disk.
In a system used at a data center or the like, where a large number of similarly configured computers operate, the contents of system disks are mostly the same but not perfectly the same. Therefore, the computers are provided with system disks that devote most of their spaces to the storage of the same information. That is why the device cost and management cost cannot easily be reduced.